Fix #1689 -- Widget: disable buy button without selected products (#1692)

src/pretix/static/pretixpresale/scss/widget.scss

@@ -56,6 +56,13 @@
         @include tab-focus;
       }
     }
+    &.disabled,
+    &[disabled],
+    fieldset[disabled] & {
+      cursor: $cursor-disabled;
+      @include opacity(.65);
+      @include box-shadow(none);
+    }
   }
   input[type="text"], input[type="number"] {
     line-height: normal;